Something that struck me is that Pandora seems very perfect is
certain ways (I'm not critising Avatar for it though). I mean
bacteria and viruses or their Pandoran equivilents must certainly
have evolved and what's stopping terran or Pandoran microbial life
from spreading from one world to the other (humans couldn't have
cured every disease by the time Avatar is set in, plus humans would
have colonized Pandora long enough for the alien microbes to learn
to infect them)? But maybe Ewya somehow doesn't permit the
evolution of infectious diseases, but that would go against
evolution...

At first glance, Pandora's psychedelic bioluminsecent forests,
exotic liforms and surreal landforms look nothing like anything on
Earth. However, Pandora shares more in common with Earth than some
would have you believe.

Bioluminescent forests on Pandora - Foxfire on Earth. Foxfire
is caused by glowing fungi and insects, and has been known to light
up entire forests. [1]

Helicodorian - Spirobranchus giganteus, a very similar tube
worm in Earth's oceans that withdraw when threatend. [2]

Actually, James Cameron has mentioned that the creatures on
Pandora had some kind of scientific background, and most have some
real-life equivilent.

Eywa - Gaia hypothesis. [3]

Watch this vid on youtube, it isn't mine, but shows some very
Pandora-like enviroments…